1. Field of the Invention
The present invention relates generally to honeycomb materials which are used widely as the core in composite panels. More particularly, the present invention relates to the resins which are used to coat the honeycomb material to increase the strength of the core.
2. Description of Related Art
Honeycomb core for composite panel applications is typically prepared from sheets of a suitable material, bonded together at controlled intervals and then expanded to form hexagonal cells. The expanded material is then cut into suitable thicknesses to produce slices which can be used as the central region of a composite panel. Commonly, the material from which such honeycomb is produced can be a non-woven paper-like sheet formed from cellulose, for example a Kraft paper, or other commonly used fibers such as high density polyethylene or other synthetic fibers. Additional materials used to make non-woven paper-like sheet include fiberglass, carbon, quartz and ceramic fibers or combinations thereof. A commonly encountered non-woven material for producing such honeycombs is based on Nomex, an aramid fibre (a poly[m-phenylene isophthalamide]) produced by E. I. duPont de Nemours (Wilmington, Del.) and supplied for purposes of honeycomb manufacture in the form of a paper-like material. Also similar are honeycombs based on mixtures of Nomex fibre and a second fibre, namely poly[p-phenylene terephthalamide], known variously as Kevlar (product of DuPont) or Twaron (product of Akzo Chemie, Wuppertal, Germany). However, whilst Nomex and the other paper-like materials are suitable basic components for honeycomb production, they do not in themselves possess sufficient strength to enable the production of strong composite panels and it is therefore common practice to coat the honeycomb with a thermosetting resin. The resin coated core may then be bonded to suitable outer skins, such as carbon or glass pre-pregs or laminates, and cured to form useful structurally strong panels. Many applications of composite panels in the aircraft industry and the like require that such panels have high strength and stiffness but are very light in weight.
Although it is possible to achieve small changes in the mechanical properties of the honeycomb by methods well known to those skilled in the art, for example by the use of alternative solvents, different cure schedules and so on, these methods generally lead to the degradation of one property at the expense of another. Typically, an improvement in the shear strength of the coated core is achieved only at the expense of a decrease in the compression strength. The reason for this effect is not fully understood, but is believed to be related to the tendency of the coating resin to deposit preferentially at either the corners or the flat portions of the honeycomb cells, modifying the compressive or shear properties respectively.
In view of the above, there is a continuing need to develop new resin coatings which can be used to strengthen honeycomb core materials. The new resin formulation must be capable of uniformly increasing strength of the honeycomb core without causing reductions in other important mechanical properties of the material.